The present invention relates to a substrate holding apparatus, and more particularly to a substrate holding apparatus suitable for use in a polishing apparatus for polishing a surface of a substrate such as a semiconductor wafer to a flat mirror finish.
Recent rapid progress in semiconductor device integration has demanded smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections. In case of photolithography which can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces of semiconductor wafers on which pattern images are to be focused by a stepper should be as flat as possible because the depth of focus of the optical system is relatively small. As a means for planarizing the semiconductor wafer, it is polished by a polishing apparatus.
Conventionally, this kind of polishing apparatus has a turntable with a polishing cloth constituting a polishing surface thereon, and a top ring for holding a substrate with a surface, to be polished, facing the turntable. The substrate is pressed against the turntable under a certain pressure by the top ring while the turntable and the top ring are being rotated, and the substrate is polished to a flat mirror finish while a polishing liquid is being supplied.
FIG. 8 is a schematic view showing main parts of an example of a conventional polishing apparatus. The conventional polishing apparatus comprises a turntable 102 with a polishing cloth 100 attached to an upper surface thereof, a top ring 104 for holding a semiconductor wafer W to be polished so as to allow the semiconductor wafer W to be rotated and to be pressed, a polishing liquid supply nozzle 106 for supplying a polishing liquid Q onto the polishing cloth 100. The top ring 104 is connected to a top ring shaft 108, which is supported by a top ring head (not shown) via an air cylinder so as to be vertically movable.
The top ring 104 comprises an elastic pad 110 of polyurethane or the like on its lower surface. The semiconductor wafer W is held in contact with the elastic pad 110 by the top ring 104. The top ring 104 further comprises a cylindrical guide ring 112 on an outer circumferential edge thereof for holding the semiconductor wafer W on the lower surface of the top ring 104 during a polishing process. The guide ring 112 is fixed to the top ring 104, and has a lower end projecting downwardly from the holding surface of the top ring 104. A recess is formed inside the guide ring 112. The semiconductor substrate W to be polished is held in the recess so as not to be removed from the top ring 104 during the polishing process.
In the conventional polishing apparatus thus constructed, the semiconductor wafer W is held against the lower surface of the elastic pad 110 which is attached to the lower surface of the top ring 104. The semiconductor wafer W is then pressed against the polishing cloth 100 on the turntable 102 by the top ring 104, and the turntable 102 and the top ring 104 are rotated to move the polishing cloth 100 and the semiconductor wafer W relatively to each other, for thereby polishing the semiconductor wafer W. At this time, the polishing liquid Q is supplied onto the polishing cloth 100 from the polishing liquid supply nozzle 106. The polishing liquid Q comprises an alkaline solution containing abrasive particles of fine particles suspended therein, for example. The semiconductor wafer W is polished by a composite action comprising a chemical polishing action of the alkaline solution and a mechanical polishing action of the abrasive particles.
In order to planarize the substrate with high accuracy by the conventional polishing apparatus, it is considered to be desirable that a substrate holding surface of the top ring 104 for holding the semiconductor substrate, and the polishing surface of the polishing cloth 100, and thus the surface of the turntable 100 to which the polishing cloth 100 is attached and which affects the polishing surface have a flatness with high accuracy, respectively.
However, during a polishing process, the top ring 104 is rotated about its own axis in sliding contact with the turntable 102 which is rotated about its own axis independently of the top ring 104. The top ring 104 is deformed by a load transmitted via the air cylinder and a complex reaction force transmitted from the polishing cloth 100 to the top ring 104, and hence the flatness of the substrate holding surface deteriorates. A part of heat generated during the polishing process is removed by the supplied polishing liquid, another part of heat is transferred to the turntable 102, and the remaining heat is transferred to the top ring 104 to increase a temperature thereof and to deform the top ring 104. Therefore, in some cases, the flatness of the substrate holding surface cannot constantly be maintained. Further, in some case, it is inevitable that the polishing rate at the central portion of the substrate is different from that at the peripheral portion of the substrate, for example, because of factors hard to be controlled, such as an amount, a position, or a temperature of the polishing liquid supplied from the polishing liquid supply nozzle.
The applicant of the present invention proposed, in the Japanese laid-open Patent Publication No. 9-225821, a substrate holding apparatus as shown in FIG. 9. In this substrate holding apparatus, a plurality of pressure spaces C1, C2, C3 defined by walls 114 are formed inside a top ring 104. In FIG. 9, the pressure spaces C1, C2, C3 are concentrically formed. A plurality of through-holes 118 communicating with both surfaces of a holding plate 116 are formed in the holding plate 116, and through-holes 120 are similarly formed in an elastic pad 110. There is provided a back pressure control mechanism capable of supplying a pressure fluid to each of the pressure spaces C1, C2, C3 while controlling the pressure of the pressure fluid. Thus, pressing forces on the backside surface of the substrate are individually controlled at each of areas on the substrate holding surface corresponding to each of the pressure spaces C1, C2, C3 for thereby pressing the substrate uniformly, thus improving the planarization of the polished surface. The numeral 122 denotes a presser ring disposed around the outer periphery of the top ring 104 for pressing the polishing cloth 100 by another pressing air cylinder different from the air cylinder for the top ring.
However, in the method utilizing a pressing force of a pressurized fluid as described above, an open space is basically formed between the backside surface of the substrate and the holding surface. Therefore, it is difficult to maintain the open space under high pressure not less than a pressure applied by the top ring. As a result, a non uniform pressing force due to the deformation of the holding plate cannot be corrected, so that the degree of the planarization of the polished surface is lowered. Similarly, since each of the areas between the backside surface of the substrate and the holding surface is not hermetically sealed, air passes through a gap produced by pressurizing the backside surface of the substrate. Therefore, it is difficult to individually control each of the pressures in the areas. Further, in the apparatus thus constructed, it is difficult to change the settings of the areas in which the back pressure is controlled depending on an actual polishing state, or to properly control the pressure in accordance with the polishing state.
The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a substrate holding apparatus which can adjust a holding state of a substrate in accordance with a polishing state, and maintain uniformity of a polishing amount over an entire surface of the substrate, or control the polishing amount intentionally.
According to the present invention as claimed in claim 1, there is provided a substrate holding apparatus for holding a substrate and pressing a surface to be polished thereof against a polishing surface on a polishing table, comprising: a holding plate having a holding surface for a substrate; a cover element for covering a backside surface of the holding plate to form a sealed back pressure space at the backside surface side; a plurality of through-holes distributed over the substantially entire holding surface for allowing the holding surface to communicate with the back pressure space; a division element removably provided for dividing the back pressure space between the holding plate and the cover element into a plurality of predetermined sealed divisional spaces in a plane; and a back pressure controller for individually controlling back pressures in the divisional spaces.
With this arrangement, a pressing force to be loaded on the substrate via an elastic pad normally becomes so uniform as to cancel an effect due to deformation of the holding plate, and hence the degree of the planarization of the polished surface can be improved. The division element preferably comprises an elastic sealing material, for example. It is easy to form the divisional spaces by disposing the division element at a predetermined position between the holding plate and the cover element, and to alter such divisional spaces. A structure for locating the division element, such as a mounting groove, may be provided in at least one of the holding plate and the cover element.
According to the present invention as claimed in claim 2, there is provided a substrate holding apparatus further comprising: a sensor for measuring a physical quantity concerned with the holding plate, and an arithmetic device for calculating an amount of deformation of the holding plate based on an output from the sensor. With this arrangement, a variation of a distribution of a pressing force applied by the holding plate in accordance with the polishing state can be estimated and adjusted to control a distribution of a polishing amount of the surface to be polished. The physical quantity may directly be measured by a strain sensor for detecting local deformation of the holding plate, or may indirectly be measured by measuring a temperature and calculating thermal deformation.
According to the present invention as claimed in claim 3, there is provided a substrate holding apparatus for holding a substrate and pressing a surface to be polished thereof against a polishing surface on a polishing table, comprising: a holding plate having a holding surface for a substrate; a cover element for covering a backside surface of the holding plate to form a sealed back pressure space at the backside surface side; a strain sensor attached to the holding plate; and a controller for adjusting a pressing force to be applied to the polishing table based on an output from the strain sensor. This arrangement can prevent the pressing force from causing excessive deformation, and create a proper polishing condition.
According to the present invention as claimed in claim 4, there is provided a substrate holding apparatus for holding a substrate and pressing a surface to be polished thereof against a polishing cloth, comprising: a holding plate having a holding surface for a substrate; a presser ring disposed around a periphery of the holding plate for pressing the polishing surface independently of the holding plate; a strain sensor attached to the holding plate; and a controller for adjusting a pressing force of the presser ring to be applied to the polishing cloth based on an output from the strain sensor. With this arrangement, a proper pressing force to cope with deterioration of the polishing cloth over time can be applied to the polishing cloth. The information on deterioration of the polishing cloth over time may be obtained by a sensor for detecting a various kind of physical quantities, and the pressing force may be controlled based on this information.
According to the present invention as claimed in claim 5, there is provided a polishing apparatus comprising a substrate holding apparatus according to any one of claims 1 to 4, and a polishing table.
According to the present invention as claimed in claim 6, there is provided a substrate holding apparatus for holding a substrate and pressing a surface to be polished thereof against a polishing surface on a polishing table, comprising: a holding plate having a holding surface for a substrate; a cover element for covering a backside surface of the holding plate to form a sealed back pressure space at the backside surface side; a plurality of through-holes distributed over the substantially entire holding surface for allowing the holding surface to communicate with the back pressure space; an O-ring and a sealing groove provided in the holding plate or the cover element for dividing the back pressure space between the holding plate and the cover element into a plurality of predetermined sealed divisional spaces; and a back pressure controller for individually controlling back pressures in the divisional spaces.